Furnace for volatilizing materials

ABSTRACT

An induction furnace for rapidly heating substances by induction to volatilize them includes a split cylindrical carbon susceptor spaced inwardly of a cylindrical outer wall and surrounding a carbon cup holding the substance for bringing the temperature of the hot zone up to as high as 2000*C. very rapidly. A metal-tometal seal is provided for sealing a tantalum rod holding the cup while permitting adjustment of the position of the cup axially of the cylindrical outer wall.

United States Patent [1 1 Stahr et al.

[ June 12, 1973 FURNACE FOR VOLATILIZING 2,673,298 3 1954 Hutchins 13/26 x MATERIALS 3,639,718 2/1972 Castonguay 219/l0.67 3,408,470 /1968 Gier 219/l0.49 [75] Inventors: Henry M. Stahr, Odgen; Andrew J.

Wunderhch, Ames, both of Iowa Primary Truhe [73] Assignee: Iowa State University Research Assistant Examiner-E Reynolds Foundation, Inc., Ames, Iowa Attorney-James [22] Filed: July 5, 1972 ABSTRACT [21 1 Appl' 269l 17 An induction furnace for rapidly heating substances by induction to volatilize them includes a split cylindrical [52] U.S. Cl 13/26, 219/10.49, 356/85 carbon s pt spa d i ard y of a cy drical outer [51] Int. Cl. 1105b 5/12 w l and surrounding a carbon cup holding the sub- [58] Field of Search... 219/ 10.49, 10.67; stance f r bringing the emperature of the hot zone up 13/26, 27; 356/85; 118/49 1 to as high as 2000C. very rapidly. A metal-to-metal seal is provided for sealing a tantalum rod holding the [5 6] I References Cited cup while permitting adjustment of the position of the UNITED STATES PATENTS cup axially of the cylindrical outer wall. 3,671,129 6/1972 Wiedeking 356/85 8 Claims, 4 Drawing Figures U 26 JJ 27 2s 300 IO 20 37 PAIENIEQMI 2873 Fig.

DRYING FLASK RECORDER ATOMIC ABSORPTION SPECTROMETER Fig. 4

FURNACE FOR VOLATILIZING MATERIALS BACKGROUND AND SUMMARY The present invention relates to induction furnaces; and more particularly, it relates to an induction furnace for rapidly heating substances to high temperatures to volatilize them. Furnaces of this type may be used for very rapid analysis for mercury or other toxic substances in biological samples. Volatilization of the sample is normally a first step in preparing the sample for mercury analysis; and sensitivity of the overall analysis system is enhanced when the furnace is capable of heating the sample to higher temperatures. In 1961, B. V. LVov used a hot carbon tube for reduction and volatilization of samples for atomic absorption analysis by passing a current through the tube for heating. The work is reported in Spectrochimica Acta, Vol. 17, page 761 (1961 Since that time, other furnaces of the same general type have been investigated. A commercial instrument has also been marketed, commonly referred to as a Massmann furnace. This furnace is described in Coloq. Spectrosc. Intern. XII Exeter (1955).

Commercial furnaces use an O-ring of synthetic rubber or other elastomeric material to seal a rod supporting a cup holding the substance to be volatilized within the furnace. This O-ring forms a seal between the rod and metal couplingsthat connect to the outer furnace cylindrical wall. It has been found that this O-ring degrades rapidly, and as the rod couples with the applied rf field, the sealing O-ring becomes hot due to the contact with the rod. As a result, the materials in the seal become volatilized, mix with the volatilized substance under analysis, and mask the resulting analysis, thereby reducing the overall sensitivity of the system.

This problem is overcome in the present system by using a tantalum rod supporting the carbon cup which holds the material under analysis and by forming a metal-to-metal seal between the tantalum rod and an end cap for the furnace. Further, the carbon cup is surrounded with a split cylindrical carbon susceptor spaced inwardly of the cylindrical outer wall of the furnace and defining the lateral boundary for the hot zone. This structure concentrates the rf energy in a zone immediately adjacent the carbon cup and enables a very rapid heating of the substance under analysis to temperatures as high as 2000C. The present system has achieved a sensitivity of 10 grams of mercury and 10" grams of lead. By using a carbon cup and a tantalum support rod, we have been able to achieve very good induction coupling to the cup and excellent corrosion stability.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in their various views.

THE DRAWING FIG. 1 is a partially broken away elevational view of a furnace construction incorporating the present invention;

FIG. 2 is a horizontal cross sectional view taken through the sight line 2-2 of FIG. 1;

FIG. 3 is a vertical cross sectional view of the metalto-metal seal between the furnace and the supporting tantalum rod; and

FIG. 4 is a functional block diagram of an overall analysis system using the present invention.

DETAILED DESCRIPTION Before describing the furnace construction in detail, the present invention may be better understood by describing the overall system for analysis in which it is used. Referring then to FIG. 4 which is a schematic functional block diagram indicating the elements of such a system, reference numeral 10 generally indicates a vaporization furnace of the type shown in FIGS. 1-3 in which a-sample for analysis is vaporized through induction heating by an rf field as quickly and efficiently as possible. The interior of the furnace 10 is purged with argon or nitrogen gas, introduced at an inlet 11 and taken from an outlet 12. The argon or nitrogen provides an inert atmosphere in the hot zone of the furnace, and it sweeps the mercury vapor (or whatever other toxic substance is being analyzed) through the system. Gases other than nitrogen or argon may be used, for example, we have shown that mercury sensitivity is equal for air, nitrogen, and argon; however, more rapid deterioration of the carbon cup which holds the sample occurs when air is used as a sweep gas. Hence, we prefer the use of argon or nitrogen.

Our work has been done primarily in the analysis of mercury as a toxic substance primarily because of the current interest in this metal, particularly in biological samples.

The metallic vapor, mixed with the purging gas, is swept through Tygon tubing schematically indicated by the line 13 and thence into a pre-filter 14, preferably comprising Millipore glass, as the filtering element. The vapor is then passed through a drying flask 15 containing magnesium perchlorate, and finally to the vapor cell of an atomic absorption spectrometer 16. In our experiments, the vapor cell was a glass tube 15 cm in length by 2 cm. in diameter with quartz windows at each end of the tube. The vapor cell was fastened to the top of a burner assembly of the atomic absorption spec trometer, thereby permitting a conventional burner mount to be used. Standard conditions for the analysis of mercury were used. The data obtained is recorded on a recorder readout system 17.

The induction furnace, presently to be described, attains a temperature of 1100C.1200C. in an activation time of 20 seconds. This compares with an activation time of over 3 minutes, as can be seen from Table I below.

TABLE I Old Susceptor New Susceptor Time Temperature Time Temperature 1.5 min. 800C. 20 sec. 1 I00 l200C. 2.0 min. 870C. 30 sec. l300 1400C. 3.0 min. 1020C. 3.5 min. 1020C.

With our system, the sensitivity of mercury in water was found to be O.0O1 l0 grams of mercury per milliliter of water. Reproducibility for a series of standard samples containing 0.0l 20 grams of mercury per milliliter of water was found to be i 1 percent relative standard deviation.

Other matrices were analyzed for mercury, including blood and tissues. Our analyses have indicated that the sensitivity of these matrices is below 0.05 10' grams of mercury per gram of sample.

Turning now to the details of the furnace, as illustrated in FIGS. 1 and 2, it includes a cylindrical side wall 19 provided with an exterior induction coil 20 excited with a source of radio frequency energy, not shown. The illustration of the coil 20 is diagrammatic in that more turns are used than the four shown.

End caps 21 and 22 are provided respectively at the top and bottom of the cylindrical outer wall 19 for sealing therewith. The end caps 21, 22 are similar, so that only one need be described in further detail. The end cap 21 includes a transverse wall 23 provided with a central exit aperture, a cylindrical side wall 24 and a peripheral flange 25 in which is machined a groove 26 for receiving an O-ring 27. The O-ring 27 is compressed in the slot 26 to form a sealing contact between the cap 21 and the outer surface of the cylindrical side wall 19 by means of a compression flange 28.

It will be observed that the O-ring 27 is located on the exterior surface of the side wall 19 and at a distance below the upper end 19a of that wall so as to be out of 20 direct communication with the interior of the furnace. This, in combination with the fact that the construction confines the hot zone of the furnace away from the wall 19 (as will be described presently) prevents contamination of the gases being analyzed by any material in the sealing O-ring 27. A similar O-ring designated 29 is provided at the inlet end of the furnace 10 to seal the lower end cap 22 with the side wall 19. These end caps are provided only to facilitate demounting of the furnace, and they can be completely eliminated in the event it is not desired to have a demountable furnace.

Located coaxially of the cylindrical side wall 19, which is preferably made from aluminum oxide, is a carbon electrode cup 30 which has an upwardly opening receptacle 30a for holding the sample. The bottom of the cup 30 is attached to a tantalum rod 31 which extends coaxially with the cylindrical side wall 19 and out through the bottom end cap 22 of the exterior of the furnace.

The cup 30 and support rod 31 are movable within the furnace 10 so that a sample may be withdrawn without taking the entire furnace apart. The movable rod 31 is, therefore, sealed to a reduced neck 22a of the bottom end cap 22 by means of a connector generally designated 33 and a reducer generally designated 34 which cooperate to effect a metal-to-metal seal between the furnace 10 and the placement rod 31 without the need for elastomeric O-rings, as described below.

Surrounding the cup 30 and extending coaxially with the cylindrical side wall 19 is a split cylindrical carbon susceptor generally designated by reference numeral 35 and including a first semi-cylindrical member 36 and a second semi-cylindrical member 37 placed together to surround the cup 30. The split susceptor 35 is spaced inwardly from the side wall 19 of the furnace so as to provide an annular region 38 through which the purging gases flow and prevent or reduce the conduction of heat from the hot zone which is defined by the susceptor 35. The hot zone extends toward the exit aperture 12 as a continuation of the cylindrical shape of the susceptor 35. The susceptor 35 is formed from carbon, and its split shape, best seen in FIG. 2, enhances the losses caused by the rf excitation energy, and it has been found to be a significant factor in the rapid heating and vaporization of the sample. The split susceptor 35 is supported and held in its coaxial position by means of an alumina tube 39 which, in turn, is held in place by means of an annular spacer member 40 which is apertured as at 41 to permit the purging gases to sweep through the annular region 38.

Turning now to FIG. 3, the connector 33 and reducer 34 which form the metal-to-metal seal between the positioning rod 31 and the furnace 10 are shown in greater detail. As will be discussed below, the connector has two separatable parts. Both a connector and a reducer are used so that the entire cup may be removed from the furnace by separating the two parts of the connector, while permitting an effective seal with the narrower rod 31 at the reducer.

The connector 33 includes first and second separatable members designated 45 and 55 respectively. The member 45 is provided with a pipe thread 46 at one end for threadedly engaging the interior of the neck 22a of the bottom end cap 22, and a screw thread extension 47, the interior of which is bored at 48, and the exterior of which is threaded as at 49. The end of the extension 47 includes a conical sealing surface 50 which engages a conical wedge member 51 backed up by a spacer 51a which has a forward conical surface 51b for engaging a corresponding rear conical surface on the back end of wedge member 51. Received within the extension 47 and wedge member 51 is a member 52 having a forward extension 53 with a smooth outer cylindrical surface. The member 55 is a hex nut which is received on the threads 47 and forces the conical wedge member 50 forwardly which, as a result, is forced to a reduced diameter by the conical surface 50 to sealingly engage the outer surface of the forward extension 53. The member 45 is provided with a central hexagonal nut for tightening the hex nut 55 onto it.

The member 52 is part of the reducer 34. The member 52 is provided with a central hexagonal nut 54 formed integrally with an externally threaded extension 54a. The end of the extension 540 is provided with a conical surface 56 which receives a complementary surface of a conical wedge member 57. The hex nut 58 is received on the extension 55 of the member 52.

interposed between the conical wedge 57 and the hex nut 58 is a flanged member 59 having a leading conical surface matching a trailing conical surface on the wedge member 57.. A similar member identified by reference numeral 51a is interposed between the conical wedge member 51 and the hex nut 55. As the hex nut 58 is tightened on the extension 54a, the flanged member 59 is urged forwardly and into sealing engagement with the rod 31 while, at the same time, urging the conical wedge member 57 into sealing engagement with the conical surface 56 of the extension 54a to effect a pressurized gas seal with the positioning rod 31 while permitting the loosening of the seal and movement or withdrawal of the rod and cup. The connector 33 and reducer 34 are commercially available under the trademark, Swagelok.

A connector generally designated by reference nu-. meral 60 in FIG. 1 and similar to the connector 33 forms a sealed connection between the end cap 21 and the outlet tube 12 of the furnace.

With the improved design of the induction furnace just described, we have attained temperatures of lC.l200C. in an activation time of 20 seconds. The rapid heating of a sample is important for a number of reasons. A rapid heating will produce sharp peaks indicative of the substance if the furnace is used with a gas chromatograph or with an atomic absorption on the other hand, produces a lower peak, broader spectrum response in the detector.

Further, with the present design there is no introduction of foreign matter from elastomeric seals, thereby yielding a greater sensitivity of the overall system. As mentioned, this is accomplished by providing a metalto-metal seal between the positioning rod 31 and the end cap 22 of the furnace. At the same time, the furnace is demountable by placing elastomeric O-rings 27, 29 on the exterior surface of the cylindrical side wall 19 and inwardly of the ends thereof so as not to be in communication with the interior of the furnace. The temperature of the furnace wall 19 adjacent the O-rings is substantially reduced by including the split susceptor 35 to concentrate the rf energy in a well-defined hot zone spaced inwardly from the side wall 19 by means of an annular region 38 through which the purging gases flow. By using tantalum for the positioning rod 31 and carbon for the sample cup 30 and the split susceptor 35, we have achieved satisfactory operation, a reduced activation time, and a reduced corrosion of the elements in the active zone while permitting manipulation of the sample cup.

Having thus described in detail a preferred embodiment of our invention, persons skilled in the art will be able to modify certain of the structure which has been illustrated and to substitute equivalent elements and materials for those which have been disclosed while continuing to practice the principle of the invention; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.

We claim:

1. An induction furnace for volatilizing materials for subsequent analysis or detection comprising: a cylindrical side wall; coil means surrounding said side wall for applying rf energy to a hot zone within said side wall; a sample cup positionably located in said hot zone; an elongated rod supporting said cup within said hot zone; metal-to-metal seal means releasably and sealingly connecting said rod to said furnace, while permitting removal of said cup and rod therefrom; carbon susceptor means surrounding said cup and spaced inwardly of said side wall to define the periphery of said hot zone;

and means for purging said furnace with a gas to sweep the volatilized sample from said hot zone.

2. The system of claim 1 wherein said sample cup is formed of carbon material and has an upwardly opening receptacle for receiving said toxic materials.

3. The system of claim 2 wherein said rod supporting said cup is tantalum.

4. The system of claim 1 wherein said carbon susceptor means includes first and second semi-cylindrical carbon members within said coil and forming a cylindrical susceptor wall around said cup and spaced inwardly of said side wall to provide an annular region; an aluminum oxide tube supporting said semicylindrical members within said furnace; and an apertured spacer member supporting said last-named tube while permitting purging gases to flow through said annular region between said cylindrical susceptor and said cylindrical side wall.

5. The system of claim 1 wherein said purging gas is argon.

6. The system of claim 1 wherein said purging gas is nitrogen.

7. The system of claim 1 wherein said furnace is adapted to prepare samples for mercury analysis in combination with an atomic absorption spectrometer.

8. A furnace for vaporizing toxic materials in preparation for atomic absorption spectrometer analysis comprising: a generally upright cylindrical side wall of aluminum oxide; first and second end caps for closing the respective ends of said side wall; first and second O-rings engaging the outer surface of said side wall and spaced inwardly of the open ends thereof to form a gas seal between the exterior surface of said tube and an associated end cap; a carbon cup adapted to place a sample for analysis in a hot zone along the axis of said tubular side wall; coil means surrounding said hot zone for applying rf energy thereto; a carbon susceptor ring including first and second separatable elements surrounding said hot zone and spaced inwardly of the side wall of said furnace for concentrating the applied rf energy in the area immediately surrounding said hot zone; a tantalum rod supporting said cup in said furnace and extending through one end cap thereof; metal-to-metal sealing means for sealing said tantalum rod to its associated end cap; and means for introducing a purging gas into said furnace and for passing said purging gas therefrom. 

2. The system of claim 1 wherein said sample cup is formed of carbon material and has an upwardly opening receptacle for receiving said toxic materials.
 3. The system of claim 2 wherein said rod supporting said cup is tantalum.
 4. The system of claim 1 wherein said carbon susceptor means includes first and second semi-cylindrical carbon members within said coil and forming a cylindrical susceptor wall around said cup and spaced inwardly of said side wall to provide an annular region; an aluminum oxide tube supporting said semi-cylindrical members within said furnace; and an apertured spacer member supporting said last-named tube while permitting purging gases to flow through said annular region between said cylindrical susceptor and said cylindrical side wall.
 5. The system of claim 1 wherein said purging gas is argon.
 6. The system of claim 1 wherein said purging gas is nitrogen.
 7. The system of claim 1 wherein said furnace is adapted to prepare samples for mercury analysis in combination with an atomic absorption spectrometer.
 8. A furnace for vaporizing toxic materials in preparation for atomic absorption spectrometer analysis comprising: a generally upright cylindrical side wall of aluminum oxide; first and second end caps for closing the respective ends of said side wall; first and second O-rings engaging the outer surface of said side wall and spaced inwardly of the open ends thereof to form a gas seal between the exterior surface of said tube and an associated end cap; a carbon cup adapted to place a sample for analysis in a hot zone along the axis of said tubular side wall; coil means surrounding said hot zone for applying rf energy thereto; a carbon susceptor ring including first and second separatable elements surrounding said hot zone and spaced inwardly of the side wall of said furnace for concentrating the applied rf energy in the area immediately surrounding said hot zone; a tantalum rod supporting said cup in said furnace and extending through one end cap thereof; metal-to-metal sealing means for sealing said tantalum rod to its associated end cap; and means for introducing a purging gas into said furnace and for passing said purging gas therefrom. 